1. Field of the Invention
The present invention relates to methods of increasing the static coefficient of friction of oleaginous compositions by adding to such compositions hydrocarbon soluble reaction products consisting of an oil soluble branched hydrocarbyl group, a linking group and a nitrogen containing polar group, which may also contain boron, oxygen or sulfur. These reaction products are useful for increasing the static coefficient of friction of oleaginous compositions, such as lubricating oils, including power transmission fluids, and particularly automatic transmission fluids, in which they are contained.
2. Description of Related Art
Transmission designs have undergone radical changes, thereby necessitating the formulation of ATF additives capable of meeting new and more stringent requirements. One change in transmission design has been the incorporation of lock-up torque converter clutches for improved fuel economy. Another change is the incorporation of 4-wheel drive systems requiring inter-axle differentiating clutches for better driveability. These two devices operate at low sliding speeds and at low energy.
The low speed low energy frictional characteristics of a lubricant are evaluated with a low velocity friction apparatus (LVFA). The LVFA apparatus uses simulated clutches approximately one inch in diameter. These small model clutches are either prepared by the clutch manufacturer to exactly duplicate production parts, or are carefully cut from full size production pieces.
These small test specimens are mounted in the LVFA test chamber and are submerged in test lubricant. An appropriate test load is then applied to the system. The machine is equipped to test at any temperature from 0.degree. C. to 200.degree. C. and once the appropriate temperature has been reached the speed of the clutch is increased from 0 to 500 rpm, and then decreased from 500 to 0 rpm. In this fashion the dependence of friction coefficient on speed and temperature can be determined over a wide range of sliding speeds and temperatures. The initial acceleration of the system from 0 sliding speed also accurately measures breakaway static friction .mu..sub.s.
An increasingly important characteristic of an automatic transmission fluid is the level of static breakaway friction that it imparts to the clutch. This parameter, expressed as breakaway static friction or .mu..sub.s, reflects the relative tendency of engaged parts, such as clutch packs or bands and drums, to slip under load. If this value is too low, the resulting slippage can impair the driveability and safety of the vehicle. This is especially important in newer cars with smaller transmissions and higher torque engines.
Chemicals which are conventionally referred to as friction modifiers can only lower the value of .mu..sub.s. This is not always desirable. Sometimes it is of great benefit to raise the level of .mu..sub.s. The products of this invention are true friction increasers, i.e., they increase the value of .mu..sub.s, without causing any deleterious effects to the fluid or transmission.
The ability to increase breakaway static friction through the use of chemical additives is extremely valuable. Previously when a transmission manufacturer needed to increase the amount of torque that could be transmitted through a locked clutch they had very few options. Conventionally, increasing the static holding capacity of a clutch has been accomplished by changing the clutch itself, either by increasing the clutch lining area (i.e. using more clutch plates), by changing the clutch lining material or by increasing pressure being applied to the closed clutch. These methods are often undesirable because they necessitate redesigning the transmission. They add weight to the vehicle, cause the transmission to take up more space and make the transmission more costly to produce. Many of these changes can also change the dynamic characteristics of the shift and make it less desirable. The products of this invention make it possible to increase the breakaway static capacity of the system without making any of these changes to the hardware.
Using only conventional friction modifiers (i.e., friction reducers) the only way to increase static friction was to reduce the level of friction reducer. This approach suffers from two problems. First, once all the friction reducer is removed the level of .mu..sub.s cannot be increased further; and second, removing the friction reducer has deleterious effects on the dynamic clutch engagement as well as the friction durability of the fluid.
In the past the only way to increase the coefficient of friction in these systems was to use a "traction fluid". These fluids however are only effective under extremely high loads and require that transmissions be extremely large and heavy to function properly. Due to their unique molecular structure these traction fluids are often very susceptible to oxidation, provide poor wear control and are not easily friction modified to give good dynamic friction characteristics. These fluids are described, for example, in U.S. Pat. Nos. 3,440,894 and 4,008,251.
No base oil alone can even approach the many special properties required for ATF service. Therefore, it is necessary to employ several chemical additives, each of which is designed to impart or improve a specific property of the fluid.
U.S. Pat. No. 4,253,977 relates to an ATF composition which comprises a friction modifier such as n-octadecyl succinic acid or the reaction product of an alkyl or alkenyl succinic anhydride with an aldehyde/tris hydroxymethyl aminomethane adduct and an overbased alkali or alkaline earth metal detergent. The ATF may also contain a conventional hydrocarbyl-substituted succinimide ashless dispersant such as polyisobutenyl succinimide. Other patents which disclose ATF compositions that include conventional alkenyl succinimide dispersants include, for example, U.S. Pat. Nos. 3,879,306; 3,920,562; 3,933,659; 4,010,106; 4,136,043; 4,153,567; 4,159,956; 4,596,663 and 4,857,217; British Patents 1,087,039; 1,474,048 and 2,094,339; European Patent Application 0,208,541(A2); and PCT Application WO 87/07637.
U.S. Pat. No. 3,972,243 discloses traction drive fluids which comprise gem-structured polyisobutylene oligomers. Polar derivatives of such gem-structured polyisobutylenes can be obtained by conversion of the polyisobutylene oligomers to polar compounds containing such functional groups as amine, imine, thioketone, amide, ether, oxime, maleic anhydride, etc. adducts. The polyisobutylene oligomers generally contain from about 16 to about 48 carbon atoms. Example 18 of this patent discloses reacting a polyisobutylene oil with maleic anhydride to form a polyisobutylene succinic anhydride which is useful as a detergent, as an anti-wear agent, and as an intermediate in the production of a hydrazide derivative. Other patents containing similar disclosures include, for example, U.S. Pat. No. 3,972,941; U.S. Pat. No. 3,793,203; U.S. Pat. No. 3,778,487 and U.S. Pat. No. 3,775,503.
While the prior art suggests a variety of additives for modifying the properties of various oleaginous compositions, there is no suggestion of any additives that are suitable for increasing the static breakaway coefficient of friction of such compositions. Accordingly, there is a continuing need for new additives and methods which would enable the formulation of oleaginous compositions, including lubricating oils and power transmission fluids, and particularly automatic transmission fluids, having increased breakaway static coefficient of friction.